(1) Field of the Invention
The present invention relates to the presentation of periodic data as a visible display and particularly to the display of results of the non-destructive testing of objects with ultrasonic energy. More specifically, this invention is directed to ultrasonic test apparatus and especially means for controlling the display of the results of testing performed with such apparatus. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
(2) Description of the Prior Art
While not limited thereto in its utility, the present invention is particularly well suited to and thus will be described in the environment of ultrasonic test apparatus. As is well known, in the non-destructive testing of objects with a beam of ultrasonic energy, echo-related signals are often displayed on an oscilloscope. The oscilloscope will include the cathode ray tube, which generates the visible display, and a number of control circuits for regulating the position and intensity of the electron beam of the cathode ray tube. Generally, the signal voltage to be measured or displayed is applied to one pair of deflection plates, usually the Y-deflection plates of the CRT, so that a positive signal voltage will produce an upward vertical deflection while a negative signal voltage will result in a downward vertical deflection of the electron beam. In order to present signals as time-dependent functions, it is necessary and customary to apply a sweep voltage which varies linearly in magnitude with time, typically a sawtooth waveform, to the horizontal or X-deflection plates of the CRT. The sawtooth voltage will result in a constant horizontal deflection speed of the electron beam. During the traversing time t.sub.H of the sawtooth voltage, the electron beam will be deflected from left to right and thereafter, during fly-back portion of the sweep voltage cycle, the beam will be returned rapidly to its starting position. During the return of the electron beam to its starting position the beam current is suppressed whereby a trace is not produced on the CRT.
When an oscilloscopic display is used in conjunction with utltrasonic testing equipment, the ultrasonic signal transmitted into the workpiece traverses a path length which may be calculated while the electron beam moves during the time t.sub.H. Accordingly, the ultrasonic signal propagation may be correlated with the movement of the electron beam. Thus, for example, if an echo signal is received by the transducer during the time that the electron beam moves to the right on the CRT, it is possible to correlate the position of the echo displayed on the CRT with the exact location of the source of the echo in the workpiece, a void or fault, for example. However, correlation of this kind is possible with precision only if the traversing time t.sub.H of the electron beam in the horizontal direction is adjusted very precisely and thereafter maintained constant over a long time span regardless of the long or short term stability of the various components of the oscilloscope. Because a full deflection of the electron beam in the CRT to the right-hand limit requires a deflection voltage of given magnitude, and also because the traversing time t.sub.H is terminated when that final amplitude is attained, the scope of the linearly rising portion of the sawtooth voltage is a function of the traversing time t.sub.H. Normally, the sawtooth voltage is generated in a sawtooth generator through the use of passive RC components. The sawtooth generator thus typically includes adjustable analog devices comprising resistors and capacitors. The electrical characteristics of these components vary as, for example, the function of age and temperature making it necessary to perform tedious recalibrations and adjustments with the aid of calibrated test objects in order to correct the substantial errors which may occur in the traversing time t.sub.H or slope of the sawtooth voltage during use of the test apparatus.